Various approaches have been developed to deal with spent alkaline stripping solutions used in the dry film imaging process of circuit board manufacture. The stripping step of dry film imaging requires that the dry film substrate that has been photopolymerized be stripped off the metal substrate. Typically, monoethanolamine and/or a similar aqueous solution(s) is used to perform this step. As a consequence, a solution containing the plastic "dry film" material and various metal components are added into the stripping solution. The problem is to remove the plastic portion from the stripping solution as well as any metal contaminants.
Three basic methods have been developed to treat the used stripper solution. The first approach is to acidify the solution to lower the pH. This precipitates the polymer portion forming a gummy residue that is difficult to remove by an automated process(es). Furthermore, the acidification, even after the precipitated polymer is removed, still contains organic material that may interfere with the separation of the metal contaminants at subsequent operations.
The second approach is to use acidic salts to lower the pH to a range of about 6. While the precipitant of the polymer is easier to deal with in this approach, and may well be automated, a great deal of solids are generated, which now need to be handled and disposed of. If the generated solids do not pass Toxicity Characteristics Leaching Procedure (TCLP) testing, the material will have to be removed as hazardous waste or material. The generation of solids, according to this second approach, is about three times the quantity of solids of the first approach.
The third approach is to use neutral salts. This approach further eases the problems associated with removing the polymer precipitate from the spent stripper solution. However, it leaves a substantial organic component in the solution that interferes with metal decontamination and forms sludge deposits within the equipment.
The waste stream resulting from the manufacture of printed circuit boards contains several constituents resulting from the application of the photoresist to the board substrate and subsequent exposure, development, stripping and washing of the substrate. For instance, in a typical process, these constituents include, e.g. metals such as copper, nickel, and lead, the dry film photoresist that has been removed from the board along with the stripper and the developer solutions. The developing and stripping solutions may be sodium carbonate, sodium hydroxide, monoethanolamine, or alcohol, for example, or any combination thereof as well one or more proprietary combinations and/or formulations, such as 4045 sold by Prata, Inc. of Manchester, N.H., for example. Commercially available fortresses include RISTON 3100 and 3300 Series fortresses (available from E. I. du Pont de Nemours and Company, Inc. under the trade names RISTON 3100 and RISTON 3300) and such components will be familiar to those skilled in the art.